let Y be non empty set ; :: thesis: for a being Function of Y,BOOLEAN holds (I_el Y) 'nor' a = O_el Y
let a be Function of Y,BOOLEAN; :: thesis: (I_el Y) 'nor' a = O_el Y
thus (I_el Y) 'nor' a = 'not' ((I_el Y) 'or' a) by Th2
.= 'not' (I_el Y) by BVFUNC_1:10
.= O_el Y by BVFUNC_1:2 ; :: thesis: verum